Connections in the mammalian central nervous system (CNS) are remarkably precise, as is exemplified in the organizaion of the vertebrate visual system. By virtue of precise connections, retinal ganglion cells confer an orderly topographic map of the visual field on to centrally located visual structures. The major aim of this proposal is to study just how these visuotopically ordered connections are established during the prenatal development of the cat's visual system. Two different aspects will be studied. One concerns the normal prenatal development of connections between retina, lateral geniculate nucleus (LGN) , and visual cortex: when do these connections form and when do they become topographically ordered? What roles fo synaptogenesis and the formation of functional connections play in this process? The other concern is with the prenatal development of visual connections in the Siamese cat. In these animals the visuotopic map contained within the LGN and visual cortex has been systematically altered due to a genetic mutation. The main question to be considered is whether the time course and pattern of development of the retino-geniculo-cortical pathway in Siamese cats are entirely different from those of normal animals. A variety of modern neuroanatomical tracing methods will be used. The development of connections between retina, LGN and cortex will be studied autoradiographically by means gestational age receiving a unilateral eye injection of tritiated amino acids. The anterograde transport of radioactively labeled materials and/or the retrograde transport of horseradish peroxidase following localized injections will be used to examine the process by which topographically ordered connections develop. An electron microcopic study of synaptogenesis will be initiated, and the presence of functional connections will be assessed by means of fetal electrophysiology.